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(#) Static Field Leaks

!!! WARNING: Static Field Leaks
   This is a warning.

Id
:   `StaticFieldLeak`
Summary
:   Static Field Leaks
Severity
:   Warning
Category
:   Performance
Platform
:   Android
Vendor
:   Android Open Source Project
Feedback
:   https://issuetracker.google.com/issues/new?component=192708
Since
:   2.2.0 (September 2016)
Affects
:   Kotlin and Java files
Editing
:   This check runs on the fly in the IDE editor
Implementation
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/LeakDetector.kt)
Tests
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/LeakDetectorTest.kt)

A static field will leak contexts.

Non-static inner classes have an implicit reference to their outer
class. If that outer class is for example a `Fragment` or `Activity`,
then this reference means that the long-running handler/loader/task will
hold a reference to the activity which prevents it from getting garbage
collected.

Similarly, direct field references to activities and fragments from
these longer running instances can cause leaks.

ViewModel classes should never point to Views or non-application
Contexts.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/test/pkg/LeakTest.java:18:Warning: Do not place Android context
classes in static fields; this is a memory leak [StaticFieldLeak]
    private static Activity sField7; // LEAK!
            ------
src/test/pkg/LeakTest.java:19:Warning: Do not place Android context
classes in static fields; this is a memory leak [StaticFieldLeak]
    private static Fragment sField8; // LEAK!
            ------
src/test/pkg/LeakTest.java:20:Warning: Do not place Android context
classes in static fields; this is a memory leak [StaticFieldLeak]
    private static Button sField9; // LEAK!
            ------
src/test/pkg/LeakTest.java:21:Warning: Do not place Android context
classes in static fields (static reference to MyObject which has field
mActivity pointing to Activity); this is a memory leak
[StaticFieldLeak]
    private static MyObject sField10;
            ------
src/test/pkg/LeakTest.java:30:Warning: Do not place Android context
classes in static fields; this is a memory leak [StaticFieldLeak]
    private static Activity sAppContext1; // LEAK
            ------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`src/test/pkg/LeakTest.java`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers
package test.pkg;

import android.annotation.SuppressLint;
import android.app.Activity;
import android.content.Context;
import android.app.Fragment;
import android.widget.Button;
import java.util.List;

@SuppressWarnings("unused")
public class LeakTest {
    private static int sField1;
    private static Object sField2;
    private static String sField3;
    private static List sField4;
    private int mField5;
    private Activity mField6;
    private static Activity sField7; // LEAK!
    private static Fragment sField8; // LEAK!
    private static Button sField9; // LEAK!
    private static MyObject sField10;
    private MyObject mField11;
    @SuppressLint("StaticFieldLeak")
    private static Activity sField12;

    private static class MyObject {
        private int mKey;
        private Activity mActivity;
    }
    private static Activity sAppContext1; // LEAK
    private static Context sAppContext2; // Probably app context leak
    private static Context applicationCtx; // Probably app context leak
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/LeakDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `LeakDetector.testStaticFields`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=192708.

(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("StaticFieldLeak")
  fun method() {
     problematicStatement()
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("StaticFieldLeak")
  void method() {
     problematicStatement();
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection StaticFieldLeak
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="StaticFieldLeak" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'StaticFieldLeak'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore StaticFieldLeak ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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